1. Field of the Invention
The invention relates to a coupling-out window for linearly polarized high-power microwaves, having at least one plate which is situated in a plate plane and is transparent to microwaves.
2. Discussion of Background
A quasi-optical gyrotron as described, for example, in the Swiss Patent CH-664,045 or in the article "Das Gyrotron, Schlusselkomponente for Hochleistungs-Mikrowellensender" [The Gyrotron, Key Component for High-Power Microwave Emitters], H. G. Mathews, Minh Quang Tran, Brown Boveri Review 6-1987, pp. 303-307, is in particular suitable for the generation of microwaves of very high power. In such a gyrotron, an alternating electromagnetic field is excited in a quasi-optical resonator, which is accommodated in an evacuated tube, in that the electrons of a beam are forced to gyrate by a strong magnetic field.
The microwaves coupled out from the resonator must be brought through a suitable microwave window out of the high vacuum of the tube into a waveguide under atmospheric conditions and thus to a load. Especially in the case of high-power gyrotrons, the coupling-out window is exposed to very great thermal and mechanical stresses. Not only must it seal off the tube in a vacuum-tight manner, but rather it must also dissipate the unavoidably absorbed power without being damaged.
In the case of continuous wave powers of 1 MW and above, and operating frequencies of typically 100-200 GHz, however, even in the case of the Al.sub.2 O.sub.3 ceramics known to be particularly suitable for dielectric windows the losses per surface would be so great that such windows would have to burst.
In principle, there are two possibilities for solving the problem of the thermal stressing of microwave windows. The first possibility consists in enlarging the window surface so that the surface stress becomes acceptable. In practice, this solution founders on account of the lack of mechanical stability of such large ceramic windows.
The second possibility, which can also be realized in practice, resides in an appropriate cooling of the plate. A double-plate window which can be cooled is known, for example, from the report "Development of the technological principles of a high-stress coupling-out window for a 200 kW long-pulse gyrotron at 140 GHz", Rudolf Bachmor, ITG Technical Report on Vacuum Electronics and Displays of the ITG Technical Conference from 8 to 10 May 1989. A coolant flows through between two round Al.sub.2 O.sub.3 ceramic plates, whereby a surface-configuration cooling is achieved.
However, even the known double-plate window does not meet the requirements which a dielectric window in the target power range of 1 MW and above must fulfil. Indeed, the transparency could be intensively improved, if the ceramics were cooled to very low temperatures (e.g. with liquid helium). However, this would involve a disproportionate additional expenditure in terms of economics.